In its most common form, Geomagnetic Induced Currents (GIC) is a well-known phenomenon that takes place as a consequence of Geomagnetic Disturbances (GMD), caused by either solar winds or intended electromagnetic pulses EMP. These shock waves can interact with the conductors of transmission and distribution circuits; such interaction causes, according to the laws of physics, the induction of currents in these circuits. Once the GIC flows in the network, it reaches the power transformers as well as the instrument transformers, shunt reactors and phase shifters connected to the transmission lines, entering through their phase connections and returning from their earthed neutral. The most important effects are related to the saturation of those apparatus' magnetic circuitry. In general, it may cause wave distortion and equipment overheating. Possible outcomes of this disturbance are the malfunction of protective systems and/or failure as well as a deterioration of the grid's performance, including voltage collapse; for a detail discussion on GIC and its effects on the power distribution system see: (1) Pirjola, R., Geomagnetically Induced Currents During Magnetic Storms; IEEE Transactions on Plasma Science, Vol. 28, Issue 6, and December 2000 pp. 1867-1873 and (2) Bozoki, B., et al. The Effects of GIC on Protective Relaying, IEEE Trans Power Delivery; Vol. 11, pp 725-739, 1996. The described phenomenon poses such a major threat to the electric power grid that it has captured attention at the highest levels of the US Federal Government. In particular it must be recognized the creation in 2002 of the EMP Commission of the US Congress devoted to assess the Threat to The US from Electromagnetic Pulse Attack-Critical National Infrastructures. A comprehensive document regarding a full discussion of the GIC problem at national level by the cited EMP Commission may be found in Report of the Commission to Assess the Threat to the US From Electromagnetic Pulse Attack-Critical National Infrastructures, April 2008.
The prior art presents alternatives to the problem of protecting the power system from the GIC phenomenon. In general it can be said that in most cases mitigation devices proposed to be inserted between the neutral of the transformer and ground, i.e. in the trajectory followed by the GIC currents; so that such an insertion produces either a reduction or plainly a total blockage of such induced currents. One example could be found in U.S. Pat. No. 8,035,935, wherein such mitigation is achieved by means of a neutral grounding resistor connected from the neutral of the transformer to ground such that those GIC currents experience a sizable reduction. This approach, while relatively simple, cost-effective and safe from the system's standpoint given the fact resistors apparatus are passive devices which can be switched on/off simply and trouble free, they can still present some shortcomings. First, the attainable reduction of GIC is relative and depends on the resistor rating, and therefore size and cost. Moreover, resistor ratings are typically based on a 10-second deployment which poses a thermal concern if the deployment requires a time period longer than 10 seconds.
A second approach is based on neutral blocking capacitors, connected from the neutral of the transformer to ground such that those GIC currents get fundamentally blocked. This approach has been discussed for two decades now and is the subject of US Patent applications in Faxvog et al, 2012-0019962 A1 and Faxvog et al, 2012-0019965 A1. The capacitor insertion, as described, becomes thus a GIC blocker because of the very low frequency (quasi DC) nature of the GIC currents. However, a real number of pitfalls can be associated to this approach; first and foremost, switching of power capacitors is well known to be quite problematic because of the huge transient currents these devices can cause compelling a need to discharge them after switching the unit off. The latter implies the requirement for several functionalities, specifically to deal with this issue. Secondly, a problem stemming from the fact that the capacitor is placed in series with the transformer Y winding, on its neutral/ground side, and therefore in series with the transformer's non-linear magnetizing reactance, thus posing a number of design challenges; notably because of the problematic proneness to ferroresonance i.e. a series LC resonant condition that can establish itself since resonant tuning is enhanced by the non-linear nature of the transformer reactance that can create, in combination with the capacitor, numerous series oscillatory natural frequencies. Hence this problem requires installing yet a resonance damping resistor in that series circuit; but placing such a resistor brings also all the shortcomings discussed above for the case of the neutral grounding resistor approach and none of its advantages. Thirdly, the presence of a capacitor may be problematic in case a power system ground fault occurs when this device is on, since the capacitor causes a transient DC offset on top of the AC fault current which impedes such current to go through a zero value, as the AC current wave does, complicating substantially the clearance of that AC fault by grid circuit breakers. In addition, capacitor units placed at transformer neutrals may produce various additional well-known hazards such as neutral instability, voltage magnification and even neutral inversion phenomena. In each case more components and functionalities must be incorporated to cope with these problems causing a major impact in cost, size and complexity. In fact the resulting many components, with convoluted functionalities, make difficult to ponder the mitigation device potential failure modes and hence making difficult to arrive at a reasonable failure rate, essential for the power system reliability impact it will cause after installation at a very sensitive power system point.
All these reasons have caused the electric utility industry to be reluctant to adopt this technology, with the end result there is to date not a single installation in the US despite the urgency to protect the electric grid from these serious threats. Thus, there is a need to improve the art of mitigation of geomagnetic induced currents (GIC) in the power systems.